1. Field of the Invention
This invention relates generally to vehicle-mounted, radio frequency antennas for use in communications and navigation, and more particularly is directed to an antenna which is incorporated into a vehicle windshield or other transparency or body panel and is operable at two different frequency bands such as in both the cellular telephone frequency band and the global positioning system (GPS) frequency band.
2. Description of the Related Art
Vehicle mounted antennas which are formed integrally in an automobile windshield or other transparency have long been used for AM and FM radio reception. Such antennas offer the advantages of low cost and an effective antenna which does not protrude from the vehicle, and consequently is not unsightly or subject to breaking. Such antennas have traditionally been formed by laminating wires or ribbon conductors of metallic film between layers of vehicle windshield glass or by additional conductors bonded to the surface of a transparency, such as the use of silver ceramic on tempered transparencies.
Growth in the use of cellular telephones and anticipated growth of electronic navigation equipment utilizing the satellites of the global positioning system have created a need for additional vehicle mounted antennas to serve the frequency bands of these systems. Traditionally, each of these systems has operated with its own discrete antenna mounted to and protruding from the exterior of a vehicle or, for portable systems, incorporated in the electronic equipment itself. Protruding cellular and GPS antennas provide good signal strength and, importantly for GPS, a wide-angle view of the sky, but create the same problems associated with protruding broadcast band antennas. Antennas mounted integrally with the electronic equipment when used from inside a vehicle provide reduced signal strength as a result of the vehicle body interposing a transmission barrier.
There is therefore a need for cellular telephone and GPS antennas which can be mounted to a surface of the vehicle, but do not protrude from the exterior of he vehicle or into its interior passenger compartment.
There is also a need for such antennas which can be inexpensively manufactured so they can be incorporated as standard equipment on all vehicles.
There is a further need for such antennas which do not alter the aesthetic or cosmetic appearance of the automobile and which require only minimal modification of existing window structures and manufacturing processes.
There is additionally a need for a single antenna of the type described above which can be used simultaneously for both cellular telephones and GPS and also exhibits a sufficiently high signal strength characteristic and gain pattern characteristics, so that it is a competitive substitute for existing, externally mounted, protruding antennas. Those characteristics are that the antennas be azimuthally omnidirectional and vertically polarized for cellular telephones and have a skyward looking, circularly polarized, horizon-to-horizon hemispheric pattern for GPS.